This invention relates to an air depolarized electrochemical battery, particularly an alkaline zinc/air battery, with a sealing member covering the air inlet port(s) in the battery housing.
An air depolarized battery is a battery that has a negative electrode that includes an active material contained within the battery and a positive electrode, called an air electrode that includes an oxygen reduction material, such as a catalyst. The catalyst reduces oxygen contained in air that enters the battery through one or more air inlet ports, producing hydroxyl ions, which migrate to the anode, where they are oxidized. The battery may contain a single air depolarized cell, or it may contain more than one cell.
An air depolarized cell can have one or more positive electrodes. One type of cell with more than one positive electrode is an air-assisted cell, which has a positive electrode containing an active material and an air electrode. When the battery is discharged, the active material in the first positive electrode can be reduced, and when the battery is at rest (or being discharged at a low rate), the air electrode can reduce oxygen in air, entering the cell to recharge the first positive electrode. An example of an air-assisted cell is disclosed in U.S. Pat. Nos. 6,383,674 and 5,079,106, which are hereby incorporated by reference.
Prior to use, an air depolarized battery is sealed from the ambient air. This is often done with a removable or puncturable sealing member placed over or within the air inlet port(s). Common types of removable sealing members include adhesive tapes and tabs, placed on an external surface of the battery housing so that they cover the air inlet port(s) during battery manufacturing, and removed by the user just prior to using the battery.
The sealing member substantially limits the flow of gases into and out of the battery. This serves a number of purposes. It minimizes the rate of entry of oxygen into the battery to prevent unnecessary consumption of the active anode material in the cell before the cell is put into use. It helps maintain the desired amount of electrolyte solvent (e.g., water) in the cell; if too much water leaves the battery, the maximum discharge reaction rate will be reduced; and if too much water enters the battery, the desired discharge reaction may conclude prematurely and the maximum discharge reaction rate can also be reduced. It also minimizes the entry of undesirable gases, such as carbon dioxide, that can cause degradation of the cell's maximum current capability and discharge capacity.
While it is desirable to substantially limit the flow of gases into and out of the battery, completely sealing the battery is undesirable for some types of air depolarized batteries. If no oxygen can enter the battery, the battery voltage drops as the oxygen initially sealed within the battery is consumed. If the voltage drops too far, it is not possible to distinguish between a good battery and a defective battery with a simple open circuit voltage test. Similarly, if the voltage drops too far the time after removal of the sealing member required for the battery to be able to sustain a suitable operating voltage can become so long that the user perceives the battery to be defective because it will not properly operate the device into which it has just been installed.
Consequently, there must be a balance between sealing the battery well enough to prevent degradation of battery discharge performance during storage and sealing the battery so we that it either cannot be distinguished from a bad battery or is mistaken for a bad battery.
In the past several approaches have been taken to achieve the desired balance between sealing air depolarized batteries well enough and sealing them too well. Examples can be found in U.S. Pat. Nos. 4,649,090 and 5,958,615, which are hereby incorporated by reference and in unexamined Japanese Patent Publication Nos. 06-260216, 06-231808, 01-151166 and 57-115771. Past attempts have included making the sealing member from a single base material or a laminate of several different materials with just the right transmission rates of various gases, but finding the optimal material or combination of materials that have suitable processing characteristics and an acceptable cost has proven to be very difficult. Various coatings have also been applied to the surfaces of sealing members, but this has also met with limited success. Adding a deoxidizer to the adhesive layer of the sealing member can reduce the oxygen transmission rate, but it may not have any effect on the transmission rates of other gases. Selecting adhesive materials based on their adhesion properties and varying the thickness of the adhesive layer of the sealing member also have their limitations, particularly in modifying the transmission rates through the sealing member (i.e., between its major surfaces), as opposed to the transmission rates through the adhesive layer, parallel to the major surfaces of the sealing member.
The battery housing has also been modified in various ways to change the rate of entry of gases into the cell without changing the sealing member. Examples can be found in U.S. Pat. Nos. 5,795,667 and 5,958,615. The number and size of the air inlet ports in the battery housing have been changed to control the rate of entry of air into the battery, and the locations of the air inlet ports have also been adjusted to improve the high rate capability of the batteries, but these factors have little effect before the sealing member is removed. The air inlet port has also been modified, so the port diameter on the outer surface is greater than the diameter on the inner surface of the container, to increase the ratio of air inlet to moisture egress before removal of the sealing member. However, this can increase the variability of the air inlet port size and shape and may also contribute to deformation of the can bottom during can manufacturing and battery closing, especially when the bottom of the can is thin.
The prior approaches, even if successful for one particular battery type and size, may not be readily adaptable to others. For example, the sealing member composition, permeability, adhesive and the like that is optimum for one battery may not be optimum for another. This can require repeating long, expensive development programs for each additional battery type and/or size.
In view of the above, an object of the present invention is to provide a sealing member that is easy and economical to manufacture and that will provide a good balance between maximizing battery storage life and maintaining an adequate battery voltage before use.
Another object of the invention is to provide a sealing member that can be readily modified for use with batteries of different types and sizes.
Yet another object of the invention is to provide a battery with air inlet port dimensions and external surfaces that are uniform and easy to manufacture and control.